1. Field of the Invention
The present invention relates to an optical-transmission-space determining apparatus for determining a state of the atmosphere where a light beam propagates and an optical-space transmission apparatus for use in telecommunication by propagating a light beam.
2. Description of the Related Art
An optical-space communication system for use in communication between a counterpart apparatus (remotely located) and an apparatus (locally located) by means of a light beam through the atmosphere as a transmission channel allows for a large capacity of data to be communicated at high speeds, is more portable than a wire communication system using, for example, an optical fiber, and also its communication channel can be easily and freely established.
In order to establish a reliable communication channel with this optical-space communication system, a light beam emitted from the locally located apparatus must not veer away from the counterpart apparatus. To this end, the diameter of the light beam must be made greater or an automatic tracking function always automatically adjusting an angle of a movable mirror so as to prevent the light beam from veering away from the counterpart apparatus must be provided.
An optical-space transmission apparatus including such an automatic tracking function is shown in FIG. 6. FIG. 6 is a perspective view of the optical-space transmission apparatus performing optical communication with the counterpart apparatus through space used as a transmission channel.
A part of a light beam emitted from the counterpart apparatus (not shown) passes through a half-mirror 140 and is incident on a four-piece light-receiving device 146. A signal in accordance with an intensity distribution of a light spot formed on a receiving surface of the four-piece light-receiving device 146 is outputted from the same, and an automatic-tracking error angle made by the optical axes of transmitting and receiving light beams is computed by an operating circuit 147 on the basis of the output signal. The operating circuit 147 sends a command signal to a movable-mirror control circuit 148 so as to make the error angle zero. The movable-mirror control circuit 148 adjusts an angle of a movable mirror 132 by simultaneously driving X- and Y-axes actuators 151. Such an optical-space transmission apparatus is disclosed, for example, in Japanese Patent Laid-Open No. 11-346192.
Upon propagating in the atmosphere, a light beam undergoes influence of a positional and temporal variation (scintillation) due to fluctuation of the atmosphere, whereby the light beam incident on the counterpart apparatus has an uneven intensity distribution. Accordingly, when a response speed of the automatic-tracking function is made higher, a tracking operation is performed on the basis of the uneven intensity distribution. As a result, irrespective of the fact that the light beam normally reaches the counterpart apparatus, the scintillation causes an emitting direction of the light beam to be vibrated, thereby leading to a tracking error of the light beam and accordingly to an increase or decrease in received-light optical power.
Since many of the optical-space transmission apparatuses are installed on the roof top of a building, indoors or the like where the apparatuses undergo a relatively small level of vibration, the movable-mirror control circuit 148 limits an automatic-tracking response characteristic in the low frequency range by arranging driving voltages Vx and Vy fed to the actuators 151 to pass through a low-pass filter unit 150. By limiting the response characteristic in the low-frequency range as described above, in which a major part of frequency components of scintillation is not included, the automatic tracking function operates so as to reduce influence of scintillation.
Unfortunately, even though many optical-space transmission apparatuses are installed on the roof top of a building, indoors or the like where the apparatuses undergo a relatively small level of vibration, some apparatuses are possibly installed at places, other than the above-mentioned ones, where the apparatuses are subjected to vibration or a pressure of window. Because of limiting a response characteristic of the automatic-tracking function only in the low-frequency range, the foregoing known optical-space transmission apparatus is incapable of responding to a quick variation such as vibration, whereby its installation place is limited.
Especially, in the night, in the morning, in the evening, or in the rainy or cloudy daytime when a level of scintillation is low, limiting the response characteristic of the automatic-tracking function only in the low frequency range is not needed. Hence, the limiting the function only in the low frequency range causes the transmission apparatus to be operated in a state in which an effect of the automatic tracking function is not fully achieved.